This invention relates to facsimile systems comprising a transmitter, a receiver and a communications network therebetween. More particularly, this invention relates to a system wherein a document is scanned in a facsimile transmitter to generate electrical information-bearing signals representing the light-dark variations in the document being scanned. These information-bearing signals are then transmitted over the communications network to a facsimile receiver where the information-bearing signals are converted to marks or images on a copy medium so as to form a copy which is a reasonable facsimile of the original document.
In several commercially available facsimile systems, the information-bearing signals which are transmitted over the communication network are FM (frequency modulated) signals. In general, the signals lie in a band between 1500 and 2400 Hz. which represents a portion of the audio range which is transmitted over ordinary telephone lines. Where this frequency range is utilized, the 1500 Hz. signal usually represents a white level, the 2400 Hz. signal represents a black level and signals in the frequency range between 1500 and 2400 Hz. represent varying degrees of gray. In the alternative, the gray scale may be eliminated or reduced such that frequencies in the lower portion of the bandwidth toward 1700 Hz. and below may represent white and frequencies in the upper portion of the bandwidth toward 2200 Hz. and above may represent black.
One low cost, and extremely effective technique for demodulation of the FM signals, involves the use of one or more single shot multivibrators. As shown in U.S. copending patent application Ser. No. 417,797 filed Nov. 21, 1973, now U.S. Pat. No. 3,911,207, and application Ser. No. 440,392 filed Feb. 7, 1974, now U.S. Pat. No. 3,916,098, a single shot multivibrator is triggered into its astable state in response to trigger pulses generated for each zero crossing of the FM signals. When the received FM signals approach the lower portion of the FM bandwidth (1500 Hz.), the trigger pulses corresponding to the zero axis crossing of the FM signals triggers or sets the single shot multivibrator to the astable state such that the astable state duration or duty cycle of the multivibrator represents a lesser portion of the multivibrator cycle which includes the stable state duration. When the frequency of the FM signals approaches the other end of the bandwidth (approximately or equal to 2400 Hz.), the astable state duration or duty cycle of the multivibrator now represents a larger portion of the multivibrator cycle. This variation in ratio in the duty cycle or astable state duration to the stable state duration may be utilized to control the writing at the facsimile receiver by detecting the average DC value from the output of the multivibrator. When the average DC value is relatively small, corresponding to the reception of 1500 Hz. signals, the average DC voltage is utilized to control the writing mechanism of the facsimile receiver so as to produce white on the copy medium. When the FM signal has a frequency of 2400 Hz. representing black, the average DC value which is relatively high is utilized to control the writing mechanism in the receiver so as to produce black on the copy medium.
Certain disturbances in the FM carrier signal may produce zero axis crossings at a rate corresponding to frequencies in excess of the 2400 Hz. which can produce errors on the copy medium. One particularly likely source of such a disturbance is acoustic ringing where an acoustic coupler is utilized between the communications network or telephone line and the receiver itself. In order to prevent the creation of white holes in the copy during the reception of black signals, the aforesaid application Ser. No. 440,392 includes means for resetting the single shot multivibrator in response to disturbances which occur while the multivibrator is in the astable state thereby reinitiating the astable state so as to prevent the formation of white holes in black copy.
As disclosed in the aforesaid application Ser. No. 440,392, the single shot multivibrator comprises a voltage comparator having one input adapted to be coupled to a source of trigger signals and another input coupled to a first tap on a voltage divider connected between a first reference voltage and a second reference voltage. When the one input to the comparator exceeds a predetermined level represented by the voltage at the first tap of the voltage divider, a flip-flop coupled to the output of the voltage comparator is reset thereby applying an appropriate control voltage to the base of a transistor which is rendered nonconductive so as to permit the timing capacitor of the single shot multivibrator to charge during the astable state. When the charge on the timing capacitor reaches a predetermined level as determined by a comparator which is connected to another tap on the voltage divider, the timing capacitor is discharged. By providing a reset means associated with the single shot multivibrator having an input indirectly connected to the input to the single shot multivibrator through a capacitor, the transistor which controls the charging of the timing capacitor may be rendered conductive in response to any disturbance which appears to be or resembles a trigger pulse applied to the input of the single shot multivibrator. This in turn reinitiates the charging of the timing capacitor so as to eliminate the creation of any white holes in black copy.
The trigger pulses which are applied to the input of the single shot multivibrator and the reset circuitry of the prior art facsimile transceivers such as that shown in the aforesaid application Ser. No. 440,392 are shown in the aforesaid copending application Ser. No. 417,797. More particularly, the trigger pulses have been generated by RC circuitry for differentiating the FM signals so as to create spikes of opposite polarity at axis crossing times of the FM signals. The output from the RC circuitry is then applied to a frequency doubling circuit comprising a single semiconductive path through the collector-emitter circuit of a transistor connected in series with a voltage dropping impedance beween a first reference voltage and a second reference voltage. The transistor is rendered conductive in response to the spikes of opposite polarities by the use of oppositely poled diodes with one connected to the base of the transistor and the other connected to the emitter of the transistor. When a positive going spike is applied to the one diode connected to the base of the transistor, the transistor is rendered conductive so as to pull the collector of the transistor toward the second voltage reference level. Similarly, a negative going spike applied to the other diode will lower the voltage at the emitter of the transistor so as to again render the transistor conductive and thereby pull the collector and input to the single shot multivibrator toward the second reference potential.
Although the above-described frequency doubling circuit performs adequately, the trigger pulses which are generated at the output and applied to the single shot multivibrator and the reset circuitry are of somewhat different width and amplitude depending upon whether a positive going spike or a negative going spike initiates the trigger pulse. In addition, the edges of the pulses generated by the frequency doubling circuit do not have a particularly steep slope. These characteristics of the trigger pulses can lead to a failure in the reset circuitry. It is for this reason that the capacitor has been provided for the indirect connection between the input to the comparator and the transistor of the reset circuitry.
U.S. Pat. No. 3,467,772 -- Crane discloses the use of a pair of parallel single shot multivibrator circuits which are triggered by pulses having a frequency double the frequency of the FM signals. However, there is no disclosure of the particular frequency doubler circuit. Moreover, there is no reset circuitry disclosed in the Crane patent which would require trigger pulses characterized by uniform width, uniform amplitude and fast or steep leading and trailing edges.